RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Dec 16 2021 at 01:04, Kevin Tian wrote:
>> From: Paolo Bonzini <paolo.bonzini@gmail.com> On Behalf Of Paolo Bonzini
>> Considering that in practice all Linux guests with AMX would have XFD
>> passthrough (because if there's no prctl, Linux keeps AMX disabled in
>> XFD), this removes the need to do all the #NM handling too.  Just make
>
> #NM trap is for XFD_ERR thus still required.
>
>> XFD passthrough if it can ever be set to a nonzero value.  This costs an
>> RDMSR per vmexit even if neither the host nor the guest ever use AMX.
>
> Well, we can still trap WRMSR(XFD) in the start and then disable interception
> after the 1st trap.

If we go for buffer expansion at vcpu_create() or CPUID2 then I think
you don't need a trap at all.

XFD_ERR:  Always 0 on the host. Guest state needs to be preserved on
          VMEXIT and restored on VMENTER

This can be done simply with the MSR entry/exit controls. No trap
required neither for #NM for for XFD_ERR.

VMENTER loads guest state. VMEXIT saves guest state and loads host state
(0)

XFD:     Always guest state

So VMENTER does nothing and VMEXIT either saves guest state and the sync
function uses the automatically saved value or you keep the sync
function which does the rdmsrl() as is.

Hmm?

Thanks,

        tglx